1. Field of the Invention
The present invention relates to an assembled battery system provided with a plurality of nonaqueous electrolyte secondary batteries connected in series, and a protection device for assembled batteries.
2. Description of the Related Art
Recently, the application of secondary batteries has been rapidly increased to various devices of high output and high voltage, such as home electronic appliances, power tools, assist bicycles, hybrid vehicles. In accordance with this, the number of series-connected batteries incorporated in a so-called assembled battery is more and more increased, and it is no longer a rare case to use ten or more secondary batteries connected in series.
For an assembled battery with a large number of secondary batteries (also called electric cells) connected in series, it is necessary to provide a protection device to prevent overcharge or overdischarge of each secondary battery when the assembled battery is charged or discharged. The combination of the assembled battery and the protection device is called an assembled battery system. In particular, in the case of an assembled battery system comprising nonaqueous electrolyte secondary batteries, a protection device having a high function, which can monitor the voltages of all electric cells, is generally employed. The protection device of high function is realized by combining various integrated circuit such as microprocessors (MPUs).
When electric cells are connected in series, variations in characteristics therebetween may well raise a problem. Variations in characteristics are, for example, those in capacity, impedance, the state of charge (SOC), etc. Among these variations, voltage variations, included in SOC variations, are most liable to cause malfunctions.
When an assembled battery system with capacity or SOC variations is left uncontrolled, it is necessary to check polarity inversion. In most assembled battery systems, part or all of the power for operating the protection device is supplied from an assembled battery incorporated therein. When the system is out of a charge/discharge state, the supply of power to each element of the protection device is limited to thereby set each element in a sleep state, in order to minimize the consumption of power. Still, it is impossible to make the consumption of power zero. Continuous flowing of imperceptible current may cause polarity inversion of an electric cell of a small capacity.
In general, when a nonaqueous electrolyte secondary battery assumes an overdischarge state or polarity inverted state, the potential of the negative electrode raises to thereby cause elusion of a metal, such as a current collector, connected to the same potential as the negative electrode, and precipitation of the metal on another element. As a result, it is possible that the battery performance will be significantly degraded, and/or the separator in the battery will be broken to cause short-circuiting. In general, once polarity-inverted nonaqueous electrolyte secondary batteries cannot be reused. In light of this, in the conventional assembled battery systems, a particular operation of, for example, setting the charge amount to approx. 50% or less is necessary before the systems are subjected to long storage. Where an assembled battery is left as it is for a long time after it is completely discharged, if it is again charged, polarity inversion may well occur. In this case, the protection device inhibits charge/discharge of the battery, and hence the battery cannot be used.
JP-A 9-92342 (KOKAI) describes that Schottky barrier diodes used as bypass circuits are connected in parallel to respective nickel-cadmium batteries connected in series, to thereby bypass the current that flows from the negative electrode side of the battery, reduced in electromotive force due to degradation, to the positive electrode side, with the result that inversion of polarity is prevented.
However, the method employed in JP-A 9-92342 is not effective for preventing the polarity inversion of a nonaqueous electrolyte secondary battery. In the method of JP-A 9-92342, even if the bypass circuits normally operate, the threshold voltages of the Schottky barrier diodes are applied as negative voltages to the respective nonaqueous electrolyte secondary batteries, whereby polarity inverted states will occur and therefore the nonaqueous electrolyte secondary batteries become unable to be used.
JP-A 2006-34085 (KOKAI) discloses a rectifier that can rectify even an imperceptible signal of a level not greater than the threshold voltage of a diode.